Tack of uncured rubber components, particularly treads, is an important property for building rubber articles, such as tires. The term "tack" as used herein refers to the ability of two uncured rubber materials or surfaces to resist separation after bringing them into contact for a short time under relatively light pressure. It is important that uncured components, such as those in a tire, especially the tread, exhibit tack so that rubber components can be securely adhered and so that splices resist separation prior to vulcanization.
Lack of sufficient tack in uncured rubber components, especially treads, has been an ongoing problem. For many decades, industry has applied cements and/or solvents to uncured rubber components in order to increase their tack. Typically, these solvents and/or cements contain hydrocarbons that can be expensive and hazardous to human health and the environment.
Examples of these cements and/or solvents are disclosed in U.S. Pat. No. 3,335,041, which issued on Aug. 8, 1967, to Alan Paul Osborne. Osborne discloses an adhesive compound that is applied as a coating to one end of a tread splice prior to joining the two ends of the splice together. U.S. Pat. No. 4,808,657, which issued on Feb. 28, 1989, to Robert J. Brown, discloses a rubbery adhesive cement that is useful to enhance the tack of rubber compounds useful to make treads.
U.S. Pat. No. 4,539,365, which issued to Chong-Kon Rhee on Sep. 3, 1985, discloses a universal cement useful for both synthetic and natural rubber compounds. U.S. Pat. No. 4,497,927, which issued to Tai et al on Feb. 5, 1985, discloses a solvent-based tire tread adhesive which provides increased green tack and rapid drying.
Efforts have been made to increase the tack of uncured undertread compounds without use of a cement and/or solvent. For example, U.S. Pat. No. 4,647,328 to Chong-Kon Rhee, which issued on Mar. 3, 1987, discloses a process for making belted tires without the use of an undertread cement. Rhee discloses use of a mixture containing p-t-alkylphenol-formaldehyde tackifying resin, a tri-methyl-dihydroquinoline polymer and a N,N'-disubstituted phenylenediamine in the undertread compound. Rhee also discloses that tread rubber should be processed in a manner that maintains the rubber at a lower temperature to prevent melting of zinc stearate in the compound and its migration or diffusion to the surface of the undertread. However, the Rhee rubber compound and process seemingly require cement to secure the tread splice.
Many rubber compounds useful to make treads contain wax as a processing aid. Waxes tend to be soluble in rubber compounds when hot; but, upon cooling, insoluble fractions tend to bleed to the compound surface. The surface coating formed by wax bleeding to the surface can protect the rubber compound from ozone attack and crack growth after cure. However, this wax coating undesirably tends to reduce tack in the uncured rubber compound. This wax bleeding (or blooming) tends to occur when waxes are added to rubber compounds in excess of their solubility.
There is a need for rubber compounds that have enhanced tack to insure splice integrity of rubber components, without the need for cements, and to insure structural integrity of uncured rubber articles.